Hydrostatic transmissions are used to transmit power from a prime mover, such as an internal combustion engine, to an output shaft for driving external devices, such as the axle of a motor vehicle. Present hydrostatic transmissions include a hydraulic pump having an input shaft which is driven by the prime mover. A hydraulic fluid is pumped from the hydraulic pump to a hydraulic motor for driving an output shaft, with no mechanical linkage between the pump input shaft and the motor output shaft.
Hydraulic pumps of the axial piston type have a cylinder block rotatable with the input shaft and pistons axially movable within the cylinder block, with the displacement of the hydraulic pump being variable and proportional to the stroke of the pistons. The pump has a tiltable swashplate which engages the pistons as the cylinder block rotates to vary the stroke of the pistons and, therefore, the displacement of the pump. By changing the tilt angle of the swashplate, a broad range of flow rates to the motor can be achieved. At the neutral position of the swashplate, the pistons are not stroked and no fluid is pumped to the motor. In this position the input shaft imparts no motion to the motor shaft. When the pump swashplate is rotated away from the neutral position, the pistons are stroked as the cylinder block rotates and hydraulic fluid is pumped to the motor.
Hydraulic motors also can be of the axial piston type. Similar to the above described variable displacement hydraulic pump, an axial piston motor has reciprocating pistons which engage a tiltable swashplate, with the stroke of the pistons determining the amount of torque to the motor output shaft from the fluid pumped to the motor. With the motor swashplate in a neutral position, the pistons are not stroked and the motor output shaft is not rotated, regardless of the displacement of the hydraulic pump. When the motor swashplate is tilted away from the neutral position, the motor pistons are stroked and the output shaft is driven by the flow of hydraulic fluid through the transmission. Thus, the output performance of the hydrostatic transmission is a function of the prescribed displacements of both the hydraulic pump and the hydraulic motor.
In the design of typical on/off road vehicles which have hydrostatic propulsion and operation of a mechanical drawbar, it often is necessary to provide a selectable gear ratio between the hydraulic motor and the drive axles to obtain the desired vehicle speed and drawbar pull, depending on the particular use being made of the vehicle. In a variable displacement hydraulic motor, the position of the motor swashplate directly determines the speed of the motor output shaft, with excess energy being provided as output torque capability of the motor. It therefore is possible to control the relationship between motor speed and motor torque which is applied to the drive axles by appropriately positioning the motor swashplate. Likewise, vehicle speed and drawbar pull are determined by the motor swashplate position.
In many situations, however, the limited displacement range of the motor swashplate is insufficient to satisfy the range of performance requirements placed on the hydrostatic transmission. In order to increase the range of performance, a gearbox is connected between the motor output shaft and the drive axles to achieve the necessary ratios of speed and torque. The requirement of a gearbox adds mechanical complexity to the overall drive train, which detrimentally impacts reliability as well as cost of the drive train. An additional problem lies in the space requirements presented by the provision of a gearbox.
One approach to varying the range of performance of a hydraulic device is shown in U.S. Pat. No. 3,093,081 to Budzich, dated Jun. 11, 1963. Budzich shows a pumping device in which a common drive shaft rotates the cylinder blocks of two variable displacement pumps. Each of the pumps has a tiltable swashplate to vary the displacement thereof. The range of pump displacements is selected so that the combined output of the pumps satisfies peak requirements of a hydraulic system. Budzich is not concerned with a transmission, however, and makes no suggestion of controlling the manner in which power is transmitted to propel a vehicle.
U.S. Pat. No. 2,562,363 to Nixon, dated Jul. 31, 1951 shows a motor unit having two separate motors. One of the motors has a positionable swashplate to vary the displacement of the motor and the other motor has a fixed displacement. The range of operation of the compound unit is limited to the displacement range of the single variable displacement motor in combination with the fixed displacement of the second motor. Further, in order to position the single variable swashplate of Nixon, an arm is directly linked to a pressure regulator mechanism and there is no capability for prescribing a desired swashplate angle.
The present invention is directed toward overcoming the problems set forth above in a novel and useful way.